共查询到20条相似文献，搜索用时 25 毫秒
1.
O. A. Druzhinin 《Izvestiya Atmospheric and Oceanic Physics》2008,44(6):768-780
The onset of a three-dimensional jet flow in a stratified fluid is studied with the aid of a direct numerical simulation. An initially cylindrical jet with a Gaussian velocity profile is considered in a fluid with stable linear density stratification. The results indicate that, if an initial small perturbation of the velocity field has a wide spectrum, an exponential growth of the isolated quasi-two-dimensional mode occurs and its spectral maximum is shifted toward smaller wave numbers in comparison with the maximum of the helical mode of the instability of a nonstratified jet. The growth rate is proportional to Ri0.5, where Ri is the global Richardson number. The onset of the instability leads to the formation of the flow’s vortex structure, which consists of a collection of different-polarity quasi-two-dimensional vortices located in a horizontal plane near the longitudinal axis of the jet. At sufficiently long times (Nt > 100, where N is the buoyancy frequency and t is time), the growth of instability reaches the saturation stage and further fluctuations in velocity and density decay under the effect of viscous diffusion. At this stage, the flow becomes self-similar and the time dependences of the transverse and vertical widths of the jet are consistent with the asymptotic behaviors of integral parameters of the flow that are observed experimentally in the far stratified wake. The results suggest that the onset of the instability of a quasitwo-dimensional mode can play the determining role in the dynamics of flow in the far stratified wake. 相似文献
2.
S. V. Shagalov V. P. Reutov G. V. Rybushkina 《Izvestiya Atmospheric and Oceanic Physics》2009,45(5):629-645
The mechanisms of the large-scale vortex structures formation in zonal jet flows (atmospheric blockings, cyclonic, and anticyclonic
vortices) is investigated. Nonlinear perturbations formed during the onset of barotropic instability of a long-wave mode in
weakly-dissipative and weakly supercritical jet flows with a symmetric velocity profile are considered in the β-plane approximation.
This analysis is performed within the framework of the asymptotic theory based on the concept of a nonlinear critical layer.
The equations describing the interaction of a wave with vorticity perturbations in a critical layer are derived. The regimes
of a quasi-stationary and nonstationary nonlinear critical layer are considered separately. Combined equations of evolution
covering the principle regimes of instability development are proposed. The existence of autowave-type structures characterized
by a balance between the energy receipt to the wave and its dissipation are obtained within the framework of a numerical simulation.
The dependence of the parameters of generated autowave structures on the shape of the zonal jet profile and the flow supercriticality
level is studied. 相似文献
3.
Ponomarev V. M. Khapaev A. A. Yakushkin I. G. 《Izvestiya Atmospheric and Oceanic Physics》2008,44(1):45-52
An approximate theory is constructed to describe quasi-two-dimensional viscous incompressible flows. This theory takes into account a weak circulation in the vertical plane and the related divergence of the two-dimensional velocity field. The role of the nonlinear terms that are due to the interaction between the vortex and potential components of velocity and the possibility of taking into account the corresponding effects in the context of the concept of bottom friction are analyzed. It is shown that the nonlinear character of friction is a consequence of the three-dimensional character of flow, which results in the effective interaction of vortices with vertical and horizontal axes. An approximation of the effect of this interaction in quasi-two-dimensional equations is obtained with the use of the coefficient of nonlinear friction. The results based on this approximation are compared to the data of laboratory experiments on the excitation of a spatially periodic fluid flow. 相似文献
4.
O. A. Druzhinin V. V. Papko D. A. Sergeev Yu. I. Troitskaya 《Izvestiya Atmospheric and Oceanic Physics》2006,42(5):615-626
The far-wake flow past a sphere towed in a fluid with high Reynolds and Froude numbers and with a pycnocline-form salt-density stratification is studied in a laboratory experiment based on particle image velocimetry and in numerical and theoretical modeling. In the configuration under consideration, the axis of sphere towing is located under a pycnocline. Flow parameters, the profiles of density and average velocity, and the initial field of velocity fluctuation in numerical modeling are specified from the data of the laboratory experiment. The fields of fluid velocity at different times and the time dependences of integral parameters of wake flow, such as the average velocity at the axis and the transverse width of the flow, are obtained. The results of numerical modeling are in good qualitative and quantitative agreement with the data of the laboratory experiment. The results of the laboratory experiment and numerical modeling are compared to the predictions of a quasi-linear and quasi-two-dimensional theoretical model. The time evolution of both the average velocity at the axis and the transverse width of the wake is obtained with the model and is in good agreement with the experimental data. The results of numerical modeling also show that, under the effect of velocity fluctuation in the wake, internal waves whose spatial period is equal to the characteristic period of the wake’s vortex structure are excited efficiently in the pycnocline. 相似文献
5.
Izvestiya, Atmospheric and Oceanic Physics - This paper investigates the stability of a jet flow with a piecewise linear velocity profile in a rotating stratified atmosphere. The linearized set of... 相似文献
6.
For the first time, the concepts of the theory of helical vortices have been applied to the Lofoten vortex of the Norwegian Sea. The estimates for azimuthal and vertical velocities have been obtained from the Massachusetts Institute of Technology general circulation model (MITgcm) for 1992–2012. The columnar vortex model with helical vorticity lines and distributions has been adapted to Scully and Rayleigh vortices. It has been shown that the vortex parameters can be determined simply from mass balance equations. The parameters of the helical vortex simulating the structure of the Lofoten vortex have been found and the radial distributions of azimuthal and vertical velocity components have been constructed. The resulting data can be interesting for an analysis of the three-dimensional structure of mesoscale vortices in the ocean. 相似文献
7.
A. E. Gledzer E. B. Gledzer A. A. Khapaev Yu. L. Chernous’ko 《Izvestiya Atmospheric and Oceanic Physics》2014,50(2):122-133
Results of experiments are considered for flows generated by different sources-sinks of mass in the rotating annular channel with beta-effect simulation using the inclined bottom. Diagrams of regimes are presented in parameters of the dimensionless angular velocity of the zonal flow averaged over the channel width and the dimensionless angular velocity of transport of vortex perturbations of cyclonic and anticyclonic types. In experiments and the simplest linear theories, most attention is paid to diagram regions with a slow motion of vortices relative to the rotating coordinate system near the parameters for stationary Rossby waves. 相似文献
8.
We studied the structure of the surface perturbations generated by a stratified flow of an ideal fluid of finite depth around underwater obstacles. We consider a cylinder modeled by a point dipole localized near the density interface both above and below the interface. It is shown that density jumps characteristic of the marine medium significantly influence the formation and variability of the structure of the surface perturbations generated during a fluid??s flowing around an underwater obstacle. The results are compared with the data of the previous model calculations made by the authors for an infinite flow around an obstacle [4]. Significant differences between them are revealed, which should be taken into account in the solution of practical problems, for example, monitoring of coastal marine basins. 相似文献
9.
It was shown based on laboratory experiments in a Large Thermally Stratified Tank (LTST) at the Institute of Applied Physics of the Russian Academy of Sciences that a turbulent axisymmetric jet in a stratified fluid with a sharp density drop (a pycnocline) intensively generates internal waves. An axisymmetric oscillation mode, for which a sufficient condition of stability in the parallel flow approximation is met, served as their source. This paper studies the stability of a nonparallel flow (with self-similar velocity profiles) that simulates a jet flow in the lower part of the pycnocline with respect to the axisymmetric mode. The estimates of the axisymmetric mode near the pycnocline are in agreement with the experimental data. The signs of the self-oscillating mode of the jet were experimentally revealed and the possibility of self-oscillations was theoretically proved: it was shown that the flow in the pycnocline vicinity is absolutely unstable. 相似文献
10.
Conductor casing jetting technique has been increasingly applied in deepwater drilling. The insight into the jetting excavation mechanisms is critical in guiding a successful conductor casing jetting operation. The real- time continuous jet excavation process is simulated with the volume of fluid (VOF) multiphase method of CFD (Computational Fluid Dynamics) ANSYS Fluent calculation software in the current study. The cohesive soil is modelled by using a kind of viscous fluid with Herschel-Bulkley model. In addition, a laboratory half round nozzle jet excavation test is designed for verification by comparison of the observed jet excavation profile with the numerical results. The sensitivity parameters affecting the conductor jetting excavation mechanism in cohesive soil are thus investigated. It is found that the application of Herschel-Bulkley (HB) model for cohesive soil and the VOF method of Fluent can provide a good simulation of jet excavation process. The maximum excavation depth can be determined by the undrained ultimate bearing capacity of the circular foundation with a bearing capacity factor of 6.7. The nozzle position, jet velocity and soil strength have significantly influence on the depth and width of the jet excavation profile in conductor oblique jet. 相似文献
11.
The local surface deformation resulting from the oblique impact of a columnar water jet has been computed, using a three-dimensional large eddy simulation, as a model of the overturning jet of a breaking wave. The emergence of the secondary jet from the front face of the initial jet has been examined and the organisation of the vortices within the jet characterised. As the secondary jet emerges, the vorticity field becomes unstable under the action of the strong shear beneath the jet surface and pairs of longitudinal counter-rotating vortices stretched along the direction of the jet projection are formed. The presence of these longitudinal vortex pairs creates convergent surface flows, resulting in the formation of longitudinal scars on the rear face of the projecting jet. Following significant growth of the scars on both its upper and lower surfaces, the jet decouples into fingers. The lateral widths of the longitudinal vortices provide a minimum measure of the finger size. A horizontal Froude number Frh, representing a measure of strength of horizontal shear in a gravity-dominated impacting flow is defined, which characterises the organisation of the longitudinal vortices occurring in the shear flow, and the resultant formation of scars and fingers. For higher Frh, stronger longitudinal vortices and deeper scars are formed at longer lateral intervals, enhancing the fingering process during the splashing event. Fundamental features of material transport in the vicinity of the surface of jets (e.g. gas transfer across a sea surface) are related to the entrainment of surface fluid by the longitudinal vortices, and is thus also characterised by Frh. 相似文献
12.
为进一步探究海洋内孤立波诱导流场对海洋工程结构物以及潜航器的影响,本文采用重力塌陷方法和粒子图像测速(Particle Image velocimetry,PIV)技术在大型分层流水槽中进行内孤立波造波以及内部流速场测量,定量分析了下凹型内孤立波诱导流场结构及其影响因素。研究表明:在密度分层流体中,PIV技术可实现对大幅面内孤立波诱导流场的精细测量以及波动结构特征的准确描述;水平流速在上下层方向相反且在跃层处最小,其剪切作用在波谷附近最强;垂向流动在波前和波后分别为上升和下沉流,两者流速值在距离波谷1/4~1/2波长位置达到最大;在相同内孤立波振幅条件下,上下层流体密度差越大、厚度比越小,则波致流场越强;随着振幅增大,流场结构与Kd V、e Kd V和MCC理论模型对应波幅适用范围的描述相吻合。 相似文献
13.
A. E. Gledzer 《Izvestiya Atmospheric and Oceanic Physics》2014,50(3):292-303
This paper presents the results of numerical calculations using shallow water equations for the currents in the laboratory experiments with a rotating circular channel. An axial symmetric function of mass source is introduced into the equations for the depth of the layer to model experimental sources and sinks of fluid, which induces opposing zonal flows together with the Coriolis force. Different configurations and amplitudes of mass sources lead to the appearance of vortex motions in the channel with different circular motions in the vortices and azimuthal displacements of their centers along the channel. Diagrams of regimes are presented in the parameters of relative angular velocities of the mean zonal flow and vortex transport around the axis of the system rotation. The differences of the theory and real experiments with currents of finite depth in a channel are discussed. 相似文献
14.
A. E. Gledzer E. B. Gledzer A. A. Khapaev O. G. Chkhetiani 《Izvestiya Atmospheric and Oceanic Physics》2017,53(6):579-591
This paper presents the results of experiments in a circular stationary and rotating channels with thin layers of conductive fluid for configurations consisting of a large number of permanent magnets and providing the MHD generation of small-scale velocity fields. The alternating radial configurations of magnets were chosen in such a way as to ensure the conservation of a discrete symmetry of their mutual arrangement relative to rotations of the circular channel around a central axis and were formed on the basis of numerical calculations with the shallow-water equations. Both in numerical and laboratory experiments, large-scale nearly circular vortices were obtained as a result of the energy transfer from the system of externally generated small-scale vortices to large-scale velocity fields (inverse cascade) under the influence of the Coriolis force in the rotating case. Single large-scale vortices and wide jet streams appear in subrotation and superrotation modes relative to external rotation, depending on its angular velocity. Rotation in a nearly circular vortex has a differential character with a decrease in the angular velocity of rotation with the radius in most area of the channel. 相似文献
15.
The flow about a circular cylinder placed centrally inside a channel is studied numerically with an unstructured collocated grid finite volume method based on the primitive variable formulation. The distance between the channel walls is allowed to vary to change the blockage ratio. Simulations are carried out over a range of Reynolds numbers that are consistent with the two-dimensional assumption. The study confirms that transition to vortex shedding regime is delayed when the channel walls are close to the cylinder because of the interaction between the vortices from the channel wall and cylinder wake. In the unsteady vortex shedding regime, the wake pattern is opposite to the classic Karman street in respect of the positions of the shed vortices. The cylinder drag coefficient and Strouhal number are considerably increased at smaller gaps while the root-mean-squared lift coefficient is significantly decreased. Several important flow parameters are correlated with the input parameters, namely Reynolds number and blockage ratio. 相似文献
16.
The problem of unsteady, laminar flow past a circular cylinder which starts translating and oscillating impulsively from rest in a viscous fluid is numerically investigated at a Reynolds number of R = 103. The flow is incompressible and two-dimensional, and the cylinder oscillations are harmonic. The transverse oscillations are only allowed when the maximum oscillatory-to-translational velocity ratio is 0.5. The investigation is based on an implicit finite difference scheme for integrating the unsteady Navier-Stokes equations together with the mass-conservation equation in their vorticity stream function formulation. A non-inertial coordinate transformation is used so that the grid mesh remains fixed relative to the accelerating cylinder. Present calculations are performed within the range of sufficiently large oscillation amplitude to induce separation. The time variation of the in-line and transverse force coefficients are presented. The study also focuses on the laminar asymmetric flow structure in the near-wake region. In this flow regime, it is found that there is alternate shedding of vortices from either side of the cylinder over an oscillation cycle (as predicted experimentally); this is the classical mode of vortex shedding leading to formation of the Kármán street. 相似文献
17.
基于PETSc FEM开源代码,采用分步有限元算法和区域分解法,并行计算了不同雷诺数下的三维顶板斜向驱动方腔流问题。计算结果表明,当顶板沿其对角线方向运动时,流体与下游侧壁发生斜向碰撞后在下游对角处汇聚并形成射流,该射流在与底面、上游侧壁碰撞后形成较为复杂的涡流结构。雷诺数大小对三维顶板斜向驱动方腔流的涡流场结构形态具有重要影响。并行性能分析表明区域分解法能有效地提高三维粘性方腔流的计算速度。 相似文献
18.
Both wind turning with height and ageostrophic flow in a stably stratified atmospheric boundary layer are analyzed using a three-parameter turbulence model. For a quasi-steady state of the boundary layer, the cross-isobaric flow is determined only by turbulent stress at the surface in the direction of geostrophic wind. The “operative” prediction models, in which the first-order turbulence closure schemes are used, tend to overestimate the boundary-layer depth and underestimate the angle between the surface and geostrophic winds when compared to “research” models (schemes of high-level turbulence closure). The true value of the angle between the surface and geostrophic winds is significant for the presentation of a large-scale flow. A nocturnal low-level jet is a mesoscale phenomenon reflected in data obtained from measurements in a stably stratified atmospheric boundary layer. It is found that such jets are of great importance in transporting humidity, momentum, and air pollution. In this study, the difference between jet flows over a homogeneous underlying surface and over a spatially localized large-scale aerodynamic roughness is shown. 相似文献
19.
The design of thrusters inspired by the locomotion of fishes is currently investigated in many research centres for unmanned underwater vehicles. Fast fishes propel themselves in water through the rhythmic motion of their tail. Propulsion is achieved by means of the periodic shedding of vortex structures by the edges of the tail. Thrust is produced because the vortices give rise to a steady jet of fluid which leaves the tail in the direction which is opposite to the forward motion of the fish. Assuming that the fish tail can be modelled by a two-dimensional plate in steady forward motion and oscillating with a combination of harmonic heaving and pitching movements, Brown and Michael’s model is presently used to determine the dynamics of the vortex structures shed by plate edges. Numerical simulations have been carried out to investigate the effects on the flow field of varying the physical parameters of the phenomenon. The knowledge of the strength and trajectory of the vortex structures shed by the plate allows the characteristics of the jet producing the thrust to be quantified. 相似文献
20.
利用运动学模型 ,研究了弯曲急流控制参数为定常和发生周期扰动时的流体质点运动轨迹以及混沌现象的产生机制。发现弯曲急流控制参数为定常时 ,流体质点的运动为规则的周期运动 ;而控制参数发生扰动时 ,可以发生流体质点在不同特征区的穿行 ,流体质点的运动具有混沌特性。弯曲急流的相速、振幅发生周期扰动均可使流体质点的运动出现混沌特性。作者从运动学上对弯曲急流区的漂流浮标混沌特性给出了解释。 相似文献